AprilAire 600 vs 700 vs 800: Bypass, Fan-Powered or Steam Humidifier?

AprilAire 600 vs 700 vs 800 at a glance

From AprilAire's Humidifier Regional Applications sheet (Form 288, 06.24), its 2025 selection guide (Form 186), its FAQ and the 700 installation template. Coverage claims are the makers' tight-house figures.

	AprilAire 600	AprilAire 700	AprilAire 800
Type	Large bypass evaporative	Fan-powered evaporative	Electrode steam
Rated output	17 gal/day	18 gal/day	11.5 to 34.6 gal/day, set by voltage and amps
Coverage claim, tight homes	Up to 5,000 sq ft (per the Amazon listing)	Up to 5,300 sq ft (AprilAire)	Up to 10,300 sq ft (per the Amazon listing)
Power	24 VAC, 0.5 A	120 VAC outlet, 0.8 A	120, 208 or 240 VAC at 11.5 or 16 A, on its own circuit
Water fed while running	3 gal/hr	6 gal/hr	0.6 to 1.8 gal/hr
Ductwork	10 x 12 3/4 in. plenum opening plus a 6 in. round bypass duct	14 3/4 x 14 5/16 in. plenum opening, no bypass duct	No plenum opening: a steam hose runs to a dispersion tube in the duct
Unit size (W x H x D)	15 3/8 x 15 3/4 x 10 1/4 in.	15 15/16 x 18 x 10 3/8 in.	10 1/8 x 20 7/8 x 7 1/8 in.
Replacement part	Water Panel #35	Water Panel #35	Steam canister #80 (#80LC for low-conductivity water)
Warranty	5 years	5 years	5 years

One figure in AprilAire's own documents disagrees: its applications sheet gives the 700 the same 3 gallon an hour feed as the 600, while the 700's installation template and AprilAire's FAQ say 6. The FAQ's drain check backs 6: a quart jar fills in about five minutes at a 600's drain and two and a half at a 700's, which is 3 and 6 gallons an hour (our arithmetic).

How the AprilAire 600, 700 and 800 add moisture

The AprilAire 600 is a bypass humidifier with no motor of its own. It mounts on the supply or return plenum, and a 6-inch round duct links it to the other side, so the furnace blower pushes warm air through its wet Water Panel and back into the system. The pressure difference between the two ducts does the work. AprilAire's sheet says bypass output falls with lower airflow and cooler air, and on heat pumps it allows bypass models only where the supply and return ducts differ by at least 0.08 in. wg and the supply air reaches at least 100°F.

The AprilAire 700 carries a built-in fan that pulls heated air from the furnace through the same #35 panel, so it needs no bypass duct. In exchange it needs a 120-volt outlet that is not on the furnace motor circuit, and its template forbids plugging it into a multi-speed blower motor. It can go on the supply plenum or the cold air return. AprilAire recommends hot water (140°F at most) for maximum output, and on a heat pump system.

The AprilAire 800 does not evaporate water from a pad. Two electrodes pass current through water in a canister and boil it, and the steam travels through a hose to a dispersion tube in the duct. Its output is set by the supply voltage and the 11.5 or 16 amp setting, not by the furnace's air temperature, which is why AprilAire's brochure points it at large homes, milder winter climates, attics, crawl spaces, closets and homes without forced-air heating. Its manual warns that the water in the canister, hose and tube can reach 212°F.

AprilAire's evaporative ratings assume the blower runs 100% of the time with 120°F plenum air and cold water, per its brochure, so a furnace that cycles on and off delivers less. Its automatic controls can run the HVAC fan for humidification without a call for heat (Blower Activation, factory set to on).

Which AprilAire size your house needs

AprilAire sizes its humidifiers with AHRI Guideline F, which it prints in its brochure and spec sheets: gallons a day by floor area (with 8-foot ceilings) and by how tightly the house is built. The coverage claims on AprilAire's boxes match the tight column. An average house needs twice that.

Gallons per day needed, AHRI Guideline F as printed by AprilAire (8 ft ceilings). The last column compares each AprilAire rating with the average-construction figure (our comparison).

Floor area	Tight	Average	Loose	Rated high enough for an average house
1,500 sq ft	5.0	10.0	15.0	500 (12), 600, 700, 720, 800
2,000 sq ft	6.7	13.4	20.0	600 (17), 700 (18), 720, 800
2,500 sq ft	8.3	16.7	25.0	600 at its limit, 700, 720 (21), 800 on 208 or 240 V
3,000 sq ft	10.0	20.0	30.1	720 (21), 800 on 208 or 240 V
4,000 sq ft	13.4	26.7	40.1	800 at 16 A on 208 or 240 V

The AprilAire 800's output depends on how it is wired, and its manual pairs each setting with a recommended home size.

AprilAire 800 capacity and recommended home size, from Table 1 of its installation and maintenance manual (8 ft ceilings)

Setting	Steam output	Tight	Average	Loose
11.5 A, 120 V	11.5 gal/day	2,500 sq ft	1,500 sq ft	1,000 sq ft
11.5 A, 208 or 240 V	20.5 or 23.3 gal/day	4,500 sq ft	2,500 sq ft	1,500 sq ft
16 A, 120 V	16.0 gal/day	3,500 sq ft	2,000 sq ft	1,500 sq ft
16 A, 208 or 240 V	30.0 or 34.6 gal/day	6,000 sq ft	4,000 sq ft	2,500 sq ft

Two things follow. On 120 volts the 800 makes 11.5 or 16.0 gallons a day, no more than a 600 evaporates, so the steam unit only pulls ahead on a 208 or 240 volt circuit. And even at its top setting the manual stops at 2,500 square feet of loose construction; a drafty older house bigger than that needs the contractor's own load calculation. The manual also says a home with more than one furnace or air handler needs a humidifier on each.

Water use: what goes down the drain

The AprilAire 600 and 700 are drain-through humidifiers. Water runs over the panel the whole time they run, and what does not evaporate carries minerals to a floor drain or condensate pump. AprilAire's FAQ says that flushing is why the panel typically needs replacing only once a year.

Feed rates from AprilAire's FAQ, applications sheet and 700 template. The evaporated and drained columns are our arithmetic at the rated output, spread over 24 hours of running.

Model	Water fed while running	Evaporated at the rating	Left for the drain
AprilAire 600	3 gal/hr	0.71 gal/hr (17 gal/day)	About 2.3 gal/hr
AprilAire 700	6 gal/hr	0.75 gal/hr (18 gal/day)	About 5.3 gal/hr

At the rating, the 700 sends more than twice as much water down the drain each hour as the 600 for one more gallon a day of output. AprilAire's HydroCore models target that waste: AprilAire says the 620's recirculation valve cuts wastewater by 55% against the 600 and the 720's by 60% against the 700. Against AprilAire models made before 2019, at rated capacity over a 5-month season, it claims savings of up to 4,650 gallons a year for the bypass 620 and up to 15,000 for the fan-powered 720.

The AprilAire 800 works differently. It fills its canister and drains part of it from time to time to hold the water's conductivity in range, using 0.6 to 1.8 gallons an hour depending on voltage, amp setting and water quality, per the applications sheet. Its manual sets out the trade-off: hard water means fewer drain cycles but more frequent canister changes, and softened water means longer canister life but more draining.

If the installer plumbs a 600 or 700 to hot water for more output, note one line in AprilAire's FAQ: these feed rates will not switch on a tankless water heater, so AprilAire recommends a conventional tank water heater for a hot-water hookup.

Which AprilAire fits your furnace or heat pump

AprilAire's Regional Applications sheet (Form 288, June 2024) lists the models it recommends for each kind of heating equipment in five US regions. The table shows its Midwest/Northeast and Mid-Atlantic/Mid-Southern zones. The Mountain and South zones match it except that hydro-air systems get only the 800, and the desert Southwest gets only the 800 for every ducted system.

AprilAire models recommended by heating equipment, Midwest/Northeast and Mid-Atlantic/Mid-Southern zones, from Form 288 (06.24)

Heating equipment	600	700	720	800
Gas or oil furnace, single or multi stage (PSC or constant-torque ECM blower)	Listed	Listed	Listed	Listed
Modulating gas or oil furnace (variable-speed ECM blower)	Not listed	Not listed	Listed	Listed
Heat pump or geothermal, single or multi stage	Listed	Listed	Listed	Listed
Modulating heat pump or geothermal	Not listed	Not listed	Listed	Listed
Boiler with air handler and hot water coil (hydro-air)	Not listed	Not listed	Listed	Listed
Air handler with no heat source	Not listed	Not listed	Not listed	Listed
Humidifying with an A/C unit	Not listed	Not listed	Not listed	Listed

The modulating rows catch buyers out. If your furnace or heat pump has a variable-speed ECM blower, AprilAire's chart leaves out both the 600 and the 700, so ask the installer before buying either. The 620 is newer than the sheet and not on it. Homes heated by a boiler, radiant floor or electric baseboard have no ducts: the sheet lists AprilAire's self-contained Model 300 and the ductless 865, an 800 with a fan pack.

Installation and wiring for each model

• AprilAire 600: a 10 by 12 3/4 inch opening in the plenum, a 6-inch round bypass duct to the other plenum, a water line, a drain and 24-volt wiring at 0.5 A. Bypass models can go on the supply or return plenum, per the applications sheet.
• AprilAire 700: a 14 3/4 by 14 5/16 inch opening, a 1/4-inch copper water line, a 1/2-inch drain hose to a floor drain or condensate pump, and a 120-volt outlet that is not on the furnace motor circuit. Its template rules out systems with more than 0.4 in. w.g. between the supply and return plenums, water pressure above 125 psi and water hotter than 140°F.
• AprilAire 800: mounted on a wall near the duct, with a dedicated circuit of at least 20 amps at the factory 11.5 amp setting or 25 amps at 16 amps. Its manual asks for water at 25 to 120 psi with a conductivity of 125 to 1,250 microsiemens per centimeter, never demineralized or reverse osmosis water, and no dispersion tube in a duct above 2 in. wg static pressure. It says the unit must be installed by qualified HVAC and electrical contractors.

AprilAire's 700 template says the warranty is void unless a qualified heating and air conditioning contractor installs the unit, and its FAQ says the warranty on any electrical component is void without a licensed HVAC installer.

Replacing an old 600 or 700? Check the opening. AprilAire's 620 spec sheet gives a 10 by 14 1/4 inch plenum opening against the 600's 10 by 12 3/4, and the applications sheet gives the 720 a 14 1/4 by 14 1/4 inch opening against the 700's 14 3/4 by 14 5/16. A like-for-like swap can reuse the hole; moving to a HydroCore model means the installer recuts or patches it (our reading of AprilAire's dimensions).

Upkeep: panels, cartridges and canisters

Replacement parts and intervals from AprilAire's 700 product page, FAQ, 620 spec sheet, 720 owner's manual and 800 manual

Model	Part	AprilAire's interval	Also
600, 700	Water Panel #35	Once or twice per heating season; typically once a year on a drain-through unit	The drain flush carries minerals away
620, 720	HydroCore Model L cartridge	Once per season, at least once a year	Hard water: new orifice every season. Soft water: orifice and valve every 3 years (720 manual)
800	Steam canister #80 or #80LC	Depends on the water: the harder it is, the sooner	Electrode wires every 3 years; an inline particulate filter can extend canister life

The #35 Water Panel fits the 600 and 700, the HydroCore Model L cartridge the 620 and 720, and the #80 canister the 800. The 800 manual calls for the #80LC where water conductivity is below 300 microsiemens per centimeter or the unit runs on 120 volts, but never with softened water.

AprilAire's FAQ also says UV light in the ductwork can leave a humidifier's base and scale control insert powdery, damage its warranty does not cover, so mount the humidifier on the opposite side of the system from any lamp. See whether HVAC UV lights work.

Automatic or manual control: the A and M versions

Each AprilAire humidifier comes with an automatic or a manual control. The automatic version uses an outdoor temperature sensor to lower the indoor target as it gets colder, limiting window condensation; the manual version (600M, 700M, 620M, 720M) is turned down by hand. AprilAire's FAQ claims up to 50% more humidity from automatic control and says any furnace-mounted model can be upgraded with its Model 60 control (Model 62 for the 800).

Either way, AprilAire's controls top out at 45% relative humidity, per its FAQ, inside EPA's advice not to humidify above 50 percent. For the range to aim for and why winter settings go lower, see the ideal indoor humidity level. If you are not yet sure dry air is the problem, start with the signs your home is too dry.

What about the AprilAire 500, 620 and 720?

AprilAire 500: the small bypass

The AprilAire 500 is the 600's smaller sibling: 12 gallons a day, a #10 Water Panel, the same 24-volt, 0.5 amp draw and 3 gallon an hour feed, and a 9 1/2 by 9 1/2 inch plenum opening. AprilAire designs it for smaller homes in moderate climates. On the AHRI table, 12 gallons covers an average house of 1,500 square feet (10.0 needed) but not 2,000 (13.4).

AprilAire 620: the water-saving bypass

The AprilAire 620 is the HydroCore version of the bypass design: 18 gallons a day (0.75 gallons an hour), 24 VAC at 0.5 A, a one-piece cartridge in place of the panel, and 55% less wastewater than the 600, per AprilAire. Its spec sheet allows 0.3 in. W.C. of static pressure with the damper wide open and 1.0 in. at damper setting 2. Choose it over the 600 when the water bill matters or the installer is cutting a new opening anyway.

AprilAire 720: the fan-powered model for modulating systems

The AprilAire 720 replaces the 700's panel with a HydroCore cartridge and a recirculation valve: 21 gallons a day, 120 VAC at 1.25 A, 60% less wastewater than the 700 and, per AprilAire, twice the evaporative efficiency on high speed. Its owner's manual says the LOW fan setting, meant for units near living space, cuts capacity by about 10%. It is the evaporative model AprilAire lists for modulating furnaces and heat pumps, and its brochure calls the 720 ideal for heat pumps and high-efficiency furnaces.

Neither older model is discontinued: AprilAire's 2025 brochure lists the 600 and 700 alongside the 620 and 720.

Who should buy which AprilAire

Buy the AprilAire 600 if

You have a single- or multi-stage gas or oil furnace, or a heat pump that does not modulate, room beside the furnace for the 6-inch bypass duct, and an average house up to about 2,500 square feet. It is the simplest install of the three: no outlet, 24 volts and one #35 panel a season. Pick the 620 instead to cut wastewater by 55%.

Buy the AprilAire 700 if

You are replacing a 700 like for like, or the furnace closet has no room for a bypass duct and the system does not modulate. It takes twice a 600's water feed for one more gallon a day. For a new fan-powered install, the 720 beats the 700 on every figure AprilAire publishes: 21 gallons a day against 18, 60% less wastewater, and a place on the chart for modulating systems.

Buy the AprilAire 800 if

Your house needs more than about 21 gallons a day (an average house over 3,000 square feet, or a loose one over 2,000), the air handler has no heat source or humidifies through an A/C unit, or you live in the desert Southwest. Plan on a 208 or 240 volt circuit, since on 120 volts it gives no more than a 600. Skip it if a 17 or 21 gallon evaporative unit covers your house: the 800 needs its own circuit, a canister matched to your water and an electrician as well as an HVAC contractor. For a single dry room rather than the whole house, see the best humidifiers.